1. Field of the Invention
The present invention concerns a pouring tube for conveying a molten metal from an upper metallurgical vessel to a lower one. In particular, it concerns a pouring tube of refractory material for the transfer of molten steel from a distributor to an ingot mold or alternatively from a pouring ladle to a distributor.
2. Description of the Prior Art
Pouring tubes designed to convey the molten metal from one metallurgical vessel to another are wear pieces highly stressed to the point that their service life could limit the pouring time. Numerous devices for introducing and changing the tube described recently in the prior art permitted resolving this problem. See, e.g., European Patents 192019 and 441927. As soon as the erosion in the vicinity of the meniscus, and even sometimes inside of the pouring tube, reaches a certain level, the worn tube is replaced with a new tube within a sufficiently short time so as not to interrupt the pouring. In these devices, a pouring tube comprises a tubular piece equipped at its upper end with a plane plate designed to slide in guides against the lower plane face either of a pouring orifice such as a nozzle or of a fixed bottom plate coupled to a mechanism for regulating the pouring jet inserted between the pouring orifice (e.g., nozzle) and the pouring tube is generally used.
These tubes can be in one piece or can consist of an assembly of several refractory pieces. In most cases, the side faces of the plate, the lower face of the plate and the upper end of the tubular part of the tube are protected by a metal case.
Despite the considerable advantages imparted to the state of the art by the systems described above and the continuous improvement that they have undergone in recent years, there are still some problems.
In particular, the appearance of cracks or microcracks is frequently observed in the pouring tube at the level of the junction between the tubular piece and the plate located at the upper end of the tubular piece. These cracks or microcracks can occur from the preheating of the tube prior to its use, the first moments of its use or during its use.
Even if these cracks have a negligible size in some cases, they must be taken into account. The passage of the molten metal in the tube induces a substantial aspiration of ambient air. Atmospheric oxygen or even nitrogen are substantial sources of contamination of the molten metal, in particular, steel. In addition, under the combined action of oxygen and very high temperatures, the refractory material can be considerably degraded at the point of arrival of oxygen, i.e., the crack. This degradation further contributes to a local deterioration of the refractory material and enlarges the crack to the point that it could be necessary to stop the pouring.
There are refractory materials more resistant to cracking. However, these materials are generally sensitive to other phenomena such as erosion, corrosion, etc. Furthermore, the cracking can be caused by a thermal shock, a mechanical shock or the application of excessively high mechanical stresses such as pressure or vibrations.
In particular, in a device for introducing and changing the tube, the plate undergoes substantial bending stresses perpendicular to its surface that may be responsible for the formation of cracks at the upper end of the tubular part. It is observed that the upper plate can be deformed by bending around an axis parallel to the direction of the guides in which the said plate slides. Thus, it is difficult to reach a compromise based only on the chemical composition of the refractory material that solves all these problems. The applicant thus undertook to find a means of reducing the formation of cracks in the pouring tube without having to modify the composition of the refractory material.
European patent application 601,700 discloses a pouring tube of refractory material comprised of a tubular element and equipped at its upper end with a plane plate. The side faces and lower faces of the said plate and the whole tubular part of this pouring tube are protected by a metal case. A reinforcing cone surrounds the upper end of the tubular part.
Such a reinforcing cone stiffens homogeneously the structure all around the upper end of the tubular part. However, as explained above, it has been observed that, in case of use in a device for introducing and changing a pouring tube, the pouring tube is subject to constraints which are not homogeneously distributed. Moreover, its large hindrance prevents the use of a cone reinforced tube such as disclosed in the European patent application 601,700 in a device for introducing and changing a pouring tube.
Other reinforcing means for refractory pouring tubes are known in the art such as for example WO 8902800 and WO 9220480, wherein the metal jacket itself is said to strengthen the tube.
The present invention thus concerns a pouring tube of refractory material comprised of a tubular element and equipped at its upper end with a plane plate, the side faces and lower face of the said plate and the upper end of the tubular part being protected by a metallic case, said plate being reinforced in its lower face by mechanical means that increase its stiffness that is characterized in that these stiffening means are comprised of a plurality of U-shaped metallic pieces, at least one being arranged on each side of the tube and attached fixedly to the part of the metal case protecting the lower face of the plate, and to the upper end of the tubular part.